Phreatic Surface Migration through an Unsaturated Levee Embankment

Levees and floodwalls providing risk reduction to major urban cities along the Mississippi River are prone to sand boils due to underlying alluvial sand foundations. This paper presents a case study for a levee embankment in Baton Rouge, Louisiana, where field measurements were used to determine the hydraulic properties of unsaturated soils incorporated into a soil-atmosphere coupled finite-element saturated-unsaturated transient seepage model. In particular, piezometer data and geoprobe cores collected from a levee embankment at Duncan Point, Louisiana, during the 2011 Mississippi River flood event were used to calibrate substratum sands and unsaturated soil embankment parameters. The results illustrate the influence of climate coupling on phreatic surface migration and pore-water pressure development. In particular, the antecedent suction pressures are more uniformly distributed within the embankment and lower in magnitude than assuming a linear interpolation of matric suction. The soil-atmosphere boundary condition was critical to recreating the in situ phreatic surface, which indicates that precipitation plays an important role in maintaining a saturated embankment even after the flood event resides.